Capacitors store electrical energy in an electric field that forms between two electrodes separated by a dielectric material. The electrodes are most commonly configured as parallel plates with fixed separation. The capacitance, C, in Farads, is directly proportional to the electrode surface area, A, usable for charge separation and inversely proportional to the distance, d, between the two parallel electrodes, as shown in equation (1), where ∈ is the permittivity of the dielectric.
                    C        =                  ɛ          ⁢                      A            d                                              (        1        )            The power rating, P, in Watts, of a capacitor cell is given by equation (2), where V is the potential difference between the two plates in volts and ESR is the equivalent series resistance in ohms.
                    P        =                              1            4                    ⁢                                    V              2                                      (              ESR              )                                                          (        2        )            The energy rating, E, in Joules, of a capacitor cell is given by equation (3).
                    E        =                              1            2                    ⁢                      CV            2                                              (        3        )            To increase the capacitance, energy and power performance of a capacitor, one has to increase ‘∈’, ‘A’, ‘V’ and/or decrease ‘d’. However the value of ‘d’ is largely determined by the intrinsic properties of the dielectric material and device working voltage and cannot be varied significantly. (The thickness of the dielectric film is determined by the required working voltage and the strength of the dielectric material, measured in volts per meter. The dielectric strength is a measure of the maximum electric field that can exist in a dielectric without an electrical breakdown.) Therefore, when high capacitance, high power and energy performance are desired, it is necessary to combine the mutual benefits achieved with a high permittivity dielectric material, an increased effective surface area, and an enhanced dielectric strength.
Capacitors may use a bulk dielectric made from an insulating material. Alternatively, capacitors may have a microscopic dielectric layer, such as a metal oxide layer. Compared to bulk dielectric capacitors, this very thin dielectric allows for much more capacitance in the same unit volume.
In electrolytic capacitors, an electrolyte and a cathode foil together form the cathode, the second “plate” of the capacitor. Most commercial electrolytic capacitors are made with aluminum or tantalum anodes. There are also ceramic-based electrolytic capacitors, as described below.
Aluminum electrolytic capacitors are used as power supplies for automobiles, aircraft, space vehicles, computers, monitors, motherboards of personal computers and other electronics. There are two types of tantalum capacitors commercially available in the market; wet electrolytic capacitors which use sulfuric acid as the electrolyte and solid electrolytic capacitors which use MnO2 as the solid electrolyte.
A typical aluminum electrolytic capacitor includes an anode foil and a cathode foil. Usually, the dielectric film is fabricated by anodizing high purity Al foil for high voltage applications in boric acid solutions. Anode, cathode and separator sheets are cut to a specific size, as per the design specification. A laminate is made up of the anode foil, the cathode foil which is opposed to the dielectric film of the anode foil, and a separator interposed between the anode and cathode foils. The laminate is wound to provide an element. The wound element is then immersed in an electrolyte, to saturate the separator, and housed in a metallic sheathed package with a cylindrical format. Here the electrolyte and the cathode foil together form the cathode. Ta and Al electrolytic capacitors fabricated using this general type of process are produced with capacitances up to 10 μF.
The capacitances of both Ta and Al based electrolytic capacitors are fairly similar. Al-based capacitors are cheaper than Ta-based capacitors on a $/farad basis, but Al-based capacitors produce current-spike noise in certain applications. On the other hand, tantalum-based capacitors are superior to Al-based capacitors in temperature and frequency characteristics and are preferred for circuits which need high stability characteristics. However, Ta metal is relatively rare and is subject to supply constraints and highly fluctuating prices. Clearly there is a need for electrolytic capacitors with performance comparable to Ta-based capacitors without having to rely on a metal that is subject to supply constraints and fluctuating prices.
Ceramic capacitors are based on the high dielectric constant rather than the electrode area. A ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. Multilayer ceramic capacitors (MLCs) typically consist of approximately 100 alternating layers of electrode and ceramic sandwiched between two ceramic cover layers. MLCs are fabricated by screen-printing of electrode layers on ceramic layers and co-sintering of the laminate. Conventionally, the electrode material is Ag—Pd and the ceramic is BaTiO3. MLCs are produced with capacitances up to tens of μF. MLCs are well suited for high frequency applications. However, MLCs have a complicated manufacturing process and that is relatively expensive.
Referring to equation (1), it may be appreciated that improvements in the performance of electrolytic capacitors are achieved by increasing the effective surface area, A, of the anode. For example, for an aluminum electrolytic capacitor this can be achieved by electrolytic etching of the aluminum substrate before anodization to form the dielectric layer at the aluminum anode surface. Further improvements in performance may be achieved by increasing ∈ by using composite dielectric layers comprising relatively large ∈ value compounds. For example, tantalum electrolytic capacitors with Ta metal anodes, polypyrrole cathodes and Ta2O5 dielectric layers have been fabricated. See M. Satoh, H. Ishikawa, K. Amane, E. Hassegawa, K. Yoshino, Syn. Metals, 71 (1995) 2259. Titania-polypyrrole nanocomposites may also be used to improve E. See J. Lin et al., Appl. Phys. Lett. 74, 2370 (1999).
Others have tried using alternate dielectric materials. For example, Chung in U.S. Pat. No. 7,144,768 describes the use of titanium and titanium alloy anodes with advantages in energy density, cost and material density when compared with tantalum. Chung states that the insulating and dielectric behavior of the titanium anode film—as measured by the leakage current and capacitance, for example—are uncertain and inconsistent and as a result, titanium and titanium alloys have generally not been used in capacitors. To overcome this limitation, Chung describes methods for controlling leakage current and capacitance in capacitors using titanium and titanium alloy anodes. Chung describes the following methods: (i) mechanical treatment, such as shot peening, of the surface to enhance the density of active sites; (ii) thermal treatment, such as quenching, to give the Ti anode an amorphous structure; and (iii) chemical treatment, such as doping the Ti or etching the Ti surface.
Having identified titanium and titanium alloys as potentially useful anode materials for capacitors there remains a need to develop a cost effective method for high volume manufacturing of titania capacitors. Furthermore, there remains a need to identify a titania capacitor structure compatible with such a cost effective method.